“We have re-evaluated, by review of the literature, how low relative humidity (RH) may influence the immediately perceived indoor air quality (IAQ), including odor, and cause irritation symptoms (i.e. longer-term perceived IAQ)”.

Authors did a literature research in major medical databases with the term “Relative Humidity” combined with a wide variety of terms like: air quality, cabin air, dry eyes, formaldehyde, inflammation, mucous membranes, offices, ozone, pungency, sensory irritation, particles, precorneal tear film, sick building syndrome, stuffy air, and VOCs. Important: “airway infection by virus” and “moisture-damaged buildings and materials” were exclusion criteria and not selected.

The impact of RH on the immediately and longer-term perceived IAQ by VOCs, ozone, and particles is complex, because both the thermodynamic condition and the emission characteristics of building materials are influenced.

Conclusion

“Most effects of humidity are ambiguous”, is the core statement of the authors, expressed in the title of the publication as “Dichotomy”.

Epidemiological, clinical, and human exposure studies indicate that low RH plays a major role in the rising frequency of reported eye irritation symptoms by alteration of the precorneal tear film. Studies indicate that RH about 40% is better for the eyes and upper airways than levels below 30%. The optimal RH may differ for the eyes and the airways regarding desiccation of the mucous membranes.

Quote: “The recommendation that “IAQ should be dry and cool" … should be considered cautiously about the development of irritation symptoms in eyes and upper airways during a workday. Studies indicate that RH about 40% is better for the eyes and upper airways than levels below 30%”.

The “dichotomy” of humidity, or rather its contrasting effects at varying levels, was already visualized in the Schofield Sterling diagram. Its "optimum zone", situated in between increasing side effects of lower and higher humidity ranges, explains why the topic is still being discussed. The assessment of the “optimum relative humidity level” varies depending on the person's standpoint and whether “fear of too much humidity” or “benefits for health” are the focus.

Wolkoff's study shows, how important it is, to maintain relative humidity above 40% in offices, where people are constantly working with visual display units.

At lower humidity, people suffer from many complaints connected to dry eyes. For contact lens wearers, it is especially troublesome.

An optimum level of between 40-60%RH prevents dehydration of the precorneal tear film and upper airways mucous membranes.